Paul Thurott on his SuperSite for Windows speaks about a new feature in Windows Vista called Automatic Disk Defragmenter. He says:

The Vista version of Disk Defragmenter includes two huge improvements over its predecessors. First, the application sports a vastly simplified user interface that does away with the colorful but pointless disk layout view, in which you could literally watch as your hard disk was graphically defragged.

Second, Disk Defragmenter can be automatically scheduled to run.Indeed, it runs automatically, by default, at 4:00 am every Sunday morning

Link